Thoroughly clean the mussels, wash them under running water. Peel and chop the onion and garlic. Clean, wash and slice the carrots, leek and celery.
Heat the butter in a large saucepan and sauté the chopped vegetables in it. Add the bay leaf and deglaze the whole thing with the white wine (we always use the Orvieto Classico as a white wine, as in my opinion it goes best with the mussels). Bring the whole stock to the boil again and put the mussels in the saucepan, cover with a lid and let them cook for about 15 minutes. The mussel shells have opened from cooking. Mussels that are still closed should not be eaten!